As will be apparent from the literature mentioned in our above-identified prior abandoned applications and the art of record in the files thereof, systems for the treatment of waste water using biodegradation so as to produce an effluent which can be discharged safely into bodies of water, and to produce sludge in sufficiently small quantities so as to enable the sludge to be treated and handled, have developed greatly in recent years.
For example, it is known to provide a plant for the treatment of waste water which includes a mechanical clarifier (rake and sand trap) for the mechanical preremoval of coarse contaminants of the waste water and contaminants which may be of an inorganic and/or nonbiodegradable nature. The precleaned waste is introduced into an activation basin which serves as a first biologically active digestion stage, then into an intermediate clarifier, into an aeration stage, into an afterclarifier basin and then into a unit for discharging and, advantageously, treating the surplus sludge generated in the process.
The waste water to be treated is generally passed in succession to the activation basin, the intermediate clarifier and the aeration stage with the waste water then being introduced into the afterclarifier basin. The biozones or biocoenoses of the activation basin and the aeration basin are separated from one another.
For the purpose of this description, the term "waste water" is intended to mean municipal or other communal sewage and/or industrial waste waters which contain sufficient contaminating organic matter to warrant biological treatment.
When we refer to a "basin" herein, we intend to include treatment vessels of any conventional design for the stated purpose and even assemblies of basins, tanks or vessels which may be physically separated from one another or in close proximity and which may be connected by ducts, overflow weirs or any other means into a single functional unit. The preferred treatment of the surplus sludge is in fermentation chambers which may be followed by driers or other (chemical) sludge processors, gasifiers, etc.
In the conventional plant for the treatment of waste water (see German patent document DE-OS No. 26 40 875 corresponding to our abandoned U.S. application Ser. No. 831,043 referred to above) the aeration stage makes use of an aeration basin with surface aerators or the like and the design and operation of this stage is such that the activation basin functions as a maximum-loading stage with a volumetric loading of about 10 kg BOD.sub.5 per cubic meter per day (kg BOD.sub.5 /m.sup.3.d). The intermediate clarifier provides a separation of the biozones and the sludge from this second activation stage (aeration) is only recycled to this intermediate clarifier or is discharged as surplus sludge.
The activation basin (aeration) thus functions as an adsorption basin in which an adsorptive, self-filtering and coagulating removal of organic compounds which are difficult to decompose is carried out.
This system has been found to be satisfactory in many cases and has been improved, as described in the aforementioned copending applications, by providing it with an aeration stage with injections of oxygen. With these systems, it is possible to obtain elimination rates of 50% to 70%, especially when the content of the compounds which are difficult to decompose in the waste water is relatively small. The activation basin can then be operated with an oxygen content of 1 to 2 mg/l (milligrams per liter) in the aerobic range.